Question: Which of these numbers is prime? ${15,\ 21,\ 65,\ 74,\ 97}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 21 are 1, 3, 7, and 21. The factors of 65 are 1, 5, 13, and 65. The factors of 74 are 1, 2, 37, and 74. The factors of 97 are 1 and 97. Thus, 97 is a prime number.